Question

A man can see distinctly from a distance of $0.5m$. If he wants to read a book placed at a distance of $25cm$, then what kind of lens should he use?

• A. Convex lens with focal length of $-50cm$
• B. Concave lens with focal length of $-50cm$
• C. Convex lens with focal length of $50cm$
• D. Concave lens with focal length of $50cm$

Answer 1

Answer: (D)

Concave lens with focal length of $50cm$

Since given that;

$u=0.5m=50cm$

$v=25cm$

To find $f$, we know that

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{50}-\frac{1}{25}=\frac{1}{f}$

$\frac{1}{f}=\frac{-1}{50}$

$f=50cm$

Hence a concave lens of focal length $50cm$ is used.